A demand for a surface light emitting element requiring low power consumption with small volume has been enhanced, and an electroluminescent element (hereinafter, referred to as an EL element) draws attention as the surface light emitting element. A constituent element of an ELD includes such as an inorganic electroluminescent element and an organic electroluminescent element (hereinafter, referred to as an organic EL element).
The organic EL element emits light in such way that high electric field is acted to emitting part, and electrons are accelerated in the high electric field to come into collision to emission center whereby the emission center is activated to make light emission, generally.
In the organic EL element, an electron and a hole are respectively injected into a light emitting layer from electron injection electrode and hole injection electrode, the organic material is raised to excited state by combination of the injected electron and hole, and light is emitted when the organic material come back from the excited state to the ground state. The organic EL element has an advantage capable of lower voltage operation than the inorganic organic EL element. It is expected to develop to a thin and flexible display and lighting usage utilizing advantage of surface light emission. White light organic EL element is expected to develop in back light of a liquid crystal display or lighting.
Several methods of constituting light emitting layer are known to produce the white light organic EL element, for example, Patent Documents 1 and 2 are cited. A method to compose the light emitting layer by plural layers containing each of BGR emitting materials, and a method compose the light emitting layer by single layer containing BGR emitting materials are known.
Positive hole is trapped by a guest material having smaller absolute value of HOMO level and electron is trapped by a guest material having larger absolute value of HOMO level in case of RGB emitting materials are incorporated in a single emitting layer. HOMO level in longer wave length material has smaller absolute value, and therefore, carrier is trapped by a longer wave length guest material.
Therefore, an amount of dopant for red light which has longer wave length should be less than the amounts of other dopants such as blue or green. The amount of dopant for red light should be less than the amounts of other dopants such as blue or green taking energy transfer into consideration, since energy transfer to longer wave length material occurs between RGB. Consequently, there is problem to deviation of chromaticity occurs due to only slight variation of mixture ratio of dopants, stability in production is insufficient.    Patent document 1: JP-A H6-207170    Patent document 2: JP-A H7-41759